REACTIONS OF SIMPLE UNSATURATED MOLECULES ...publications.iupac.org/pac-2007/1975/pdf/4401x0043.pdfREACTIONS OF SIMPLE UNSATURATED MOLECULES WITH THE POLYNUCLEAR CARBONYLS OF SOME

REACTIONS OF SIMPLE UNSATURATEDMOLECULES WITH THE POLYNUCLEAR

CARBONYLS OF SOME GROUP VIII ELEMENTS

J. LEWIS and B. F. G. JOHNSON

Chemical Laboratory, University of Cambridge, UK

ABSTRACT

The reactivity of polynuclear metal complexes involving metal—metal bondswith simple inorganic and organic molecules is considered. The basic structuralforms of polynuclear aggregates is reviewed and the fiuxional nature of poly-nuclear carbonyls is surveyed. The use of '3C n.m.r. spectroscopy in deter-mining the mechanism of fiuxional interchange within the polynuclearcarbonyls of Group VIII elements is described and the variety of pathwaysfor exchange of carbonyl groups on a given metal centre or between differentmetal centres is established and correlated with the structure of the polynuclearspecies.

As the majority of structures of polynuclear carbonyls are based on triangularfaced polyhedra, the dodecacarbonyls of iron, ruthenium and osmium havebeen used as the simplest model for considering the interaction of inorganicand organic molecules with a polynuclear aggregate. The general facility ofpolynuclear metal species in accepting or donating electrons (i.e. electronicamphoteric behaviour) is related to their orbital structure and their readyinteraction, involving a variety of bonding patterns, with simple inorganic mole-cules is reviewed. The considerable facility for hydrogen abstraction isemphasized and the chemistry of the substituted hydridocarbonyls described.For simple alkenes and alkynes, a number of bonding patterns have beenestablished, with bonding to two or three metal atoms of the cluster unit. Foralkenes, interaction occurs with fission of carbon—hydrogen bonds and theformation of metal—hydrogen and metal—carbon bonds. The relationshipbetween the coordinated organic fragment and the product of interaction of thepolynuclear groupings with alkynes is established. An analogy between thebehaviour of these unsaturated species with polynuclear carbonyls and the

interaction of alkenes and alkynes with metal surfaces is emphasized.

The reactivity of organic species coordinated to a metal centre has been apoint of considerable activity over the past decade. These studies haveillustrated that major modifications in the stability and structure of organicgrouping may occur on coordination to a metal centre. As far as the structureand reactivity are concerned studies of this kind do pose the question ofhow far the coordination of a metal ion to an organic group may be viewedas a minor perturbation of an organic system, and this seems especially truewhen organic it electron systems are involved in the bonding pattern.

43

J. LEWIS AND B. F. G. JOHNSON

The extension of these studies to include the interaction of an unsaturatedmolecule with more than one metal centre has not been extensively investi-gated. Such studies that are available, however, suggest, especially in systemswhere the metal centres are part of a metal—metal bonding network, thatthey may provide potential analogies for the study of the structure andreactivity of organic species on metal surfaces or with heterogeneous catalyticsystems. The nature of the bonding of even simple organic molecules, suchas ethylene on a metal surface, is not well understood and has only recentlybeen subjected to detailed studies by techniques such as ESCA1. One of themain lines of structural evidence available arises from studies on the changein the infra-red spectra of the species on interaction with a metal surfaceand has established the formation of metal—hydrogen, metal—carbon inter-actions and a major modification to the it-electron distribution within thecarbon—carbon bond2. It is of obvious interest to establish how simpleorganic molecules interact with a multimetal system, and contrast this be-haviour with that of a single metal centre, although the obvious danger ofdrawing too close an analogy with metal surfaces must be recognized. Thispaper sets out to establish some of the variation in bonding patterns and toconsider the general chemistry of both simple inorganic and organic moleculeswith cluster units in which metal—metal bonding occurs.

A convenient system to illustrate these points is provided by some of theGroup VIII carbonyls. The systems discussed will be mainly the trinucleardodecacarbonyls of iron, ruthenium and osmium, as these provide thesimplest unit in which a multibonding centre may be expected. The mainemphasis of the discussion will be directed towards three aspects of the chem-istry, viz.

(i) The nature of the bonding and the fluxionality exhibited by the poly-nuclear carbonyls of Group VIII; /

(ii) The variation in the reactivity of the trinuclear clusters to inorganicdonors;

(iii) The structure and reactivity of simple unsaturated organic groups withthe polynuclear carbonyls.

Obviously it is not possible to cover all aspects of these subjects in detail,but it is hoped that the following will provide an outline of some of the moreimportant features of this chemistry.

(I) STRUCTURE AND FLUXIONALITY OF POLYNUCLEARMETAL CARBONYLS

(a) StructureThe known polynuclear carbonyls of the Group VIII elements are given

in Table i. The preparation of the higher polynuclear carbonyls of osmiumwas established recently and involves the thermal decomposition of thetrinuclear dodecacarbonyl, which gives the required carbonyls in highyield4.

0s3(CO)12 Os5 (CO)16 + 0s6(CO)18 + 0s7(CO)21 + 0s8(CO)23The production of these carbonyls illustrates the important property of the

44

POLYNUCLEAR CARBONYLS OF GROUP VIII

polynuclear carbonyls of the second and third row elements of yieldinghigher polynuclear aggregates on controlled thermal decomposition. Theidentification of species of this nature is primarily accomplished from massspectroscopic techniques and imposes, at the moment, a limitation to theestablishment of clusters of much greater size.

Table 1. Carbonyls of Group VIII elements

Fe(CO)5Fe2(CO)9Fe3(CO)12

Co2(CO)8 Ni(CO)4Co4(CO)12Co6(CO)16

Ru(CO)5Ru3(CO)12

[Rh2(CO)8] —Rh4CO)12Rh6(CO)16

Os(CO)50s2(CO)90s3(CO)120s5(CO)16Os6(CO)180s7(CO)21Os8(CO)23

Ir4CO)12 —Ir6(CO)16

The rationalization of the structure of these carbonyl clusters provides agood example of the necessity to consider the cluster unit in toto, rather thanthe individual electronic requirements of single metal atoms. The currentrationale of the structures of simple polynuclear carbonyl systems is basedon the donation of two electrons by each carbonyl group to the metal clustersystem and the attainment of the inert gas configuration for each metal atomwithin the cluster by the formation of metal—metal bonds. This process hasconsiderable success in predicting the formula and structural details of boththe carbonyls and their derivatives. The first real ambiguity arose in estab-lishing the most stable electronic structure for the [Rh6(CO)16] cluster unit,which was designated5 as corresponding to an 84, 86 or 88 electron system,i.e. the units [Rh6(CO)16]2 , Rh6(CO), [Rh6(CO)16]2 —. For the morecomplex systems it is also possible to consider more than one structuraltype in which each individual metal atom fuffils the inert gas rule, by varyingthe nature and number of electrons involved in the metal—metal bond froma given metal atom.

A convenient correlation of the potential structures has been suggestedby Wade, based on the number of electrons involved in bonding within themetal cluster in a manner used to rationalize the structure of the higherboron hydride derivatives6. The basic analogy between the two fields arisesfrom the utilization of three hybrid orbitals to the cluster bonding system,by each metal atom, as postulated for the boron hydrides series and theemployment of the remaining six bonding orbitals of the initial nine orbitals(5d + is + 3p) on each metal atom for bonding ligand orbitals. The orbitalsavailable for ligand bonding are then filled and the electron pairs remainingare considered to be involved in cluster bonding. The basic stereochemistryof the cluster is established from the number of electron pairs involved in the

45


cluster bonding, such that (n + 1) pairs are associated with regular polyhedrawith n vertices; thus, five skeletal bonding pairs are associated with a tetra-hedral arrangement, six with a trigonal bipyramid and seven with a regularoctahedron. For a system in which the number of skeletal electron pairs isthe same as the number of metal atoms in the cluster, the structure is related tothe metal cluster of one metal atom less and one face of the polyhedra iscapped by the remaining metal atom. Using the above procedure, both0s5(CO)16 and 0s6(CO)18 involve six skeletal bond pairs. The predictedstructure of the 0s6(CO)18 species on the Wade theory would, therefore, bebased on a trigonal bipyramid with one face capped by an osmium unit,i.e. an octahedron of C2 symmetry. The regular octahedral symmetry, °h'which would be consistent with the inert gas configuration, would not beexpected. The addition of two electrons to the 0s6(CO)18 unit would increasethe cluster bonding electron pairs to seven and Wade's theory would predictoctahedral symmetry, °h' for the resulting ion [0s6(CO)1 8]2 —.

In agreement with the above, the structure of the compound 0s6(CO)18,has been shown7 to involve a capped trigonal bipyramid (Figure 1). Reactionof the complex with activated zinc yields the anion [0s6(CO)1 8] whichhas a very simple infra-red spectrum consistent with octahedral symmetry,Oh8. The structure of the carbonyl 0s7(HO)21 is also consistent with this

46

Figure 1


theory, the structure being established as an octahedron of osmium atomsof °h symmetry, with one face capped by the remaining osmium atom7.These structural data emphasize the sensitivity of the basic skeletal structuresof the metal systems in these polynuclear carbonyls to the electronic structureof the cluster bonding system and also imply a facility for these systems toaccept and donate electrons from the skeletal metal framework, which mayrelate to pure metal systems. The facility of polynuclear aggregates to act aselectron 'sinks' has been well recognized, and in particular the studies ofDahi and co-workers have established this electronic amphoteric behaviourfor a variety of trinuclear and tetranuclear metal clusters; extensive structuralstudies on the variation in metal—metal bond distances and geometry havebeen rationalized in terms of the electron acceptor—donor power of thecluster fl592•

Although Wade's theory is empirical in nature, it has a general utility indealing with structural data in this type of compound. The theory has beengiven some mathematical foundation by Mingos who has shown, using aWolfsberg— Helmholtz approximation, that for the carbonyl anion [Co6-(CO),4]2 the metal cluster orbitals have a very high proportion of s and pcharacter being of a similar character to that employed in the B6H - ion.This work'3 also emphasizes that the highest filled molecular orbitals are

,weakly antibonding with regard to the metal—metal frameworkt.

(b) FluxionalityPrevious work, particularly that using infra-red and 'H n.m.r. spectroscopy,

has illustrated that many carbonyls and substituted carbonyls have struc-tures in solution which differ significantly from that observed in the solidstate'4' 15 In many cases, in solutions, an equilibrium exists between anumber of different structural forms. In addition, the fluxional nature ex-hibited by many organic molecules coordinated to metal carbonyl fragmentshas been established primarily from the study of 31P and 'H n.m.r. spectro-scopy. With the advent of '3C n.m.r. spectroscopy it has become pos-sible to study the related problem for the metal carbonyl and, in particular, toestablish the role of the carbonyl groups in the mechanism of these fluxionalprocesses. The following account emphasizes the dynamic nature of mole-cules in solution and the ready mobility, on the n.m.r. time scale, of carbonylgroups within these molecules.

it is perhaps pertinent at this stage to emphasize two differences between'H and '3C n.m.r. spectroscopy. As the nuclear relaxation times of the twonuclei may differ considerably, in contrast to the behaviour for protonsystems the probability of using the intensity of the '3Csignal as a measureof relative concentration of carbonyls within different environments isdifficult. In particular the carbon signal is very sensitive to the nature of thenuclei directly bonded to the carbon and, in unfavourable circumstances, itis possible that no signal will be detected. This problem has been overcome,in part, by the use of paramagnetic relaxation reagents, such as chromium

t The essence of the 'Wade' theory is that the highest filled and lowest empty molecularorbitals are primarily cluster bonding orbitals, with the metal—ligand bonding orbitals at verylow energy.

47

Com

poun

d (te

mp.

, 'C

)

Trm

ucle

ar co

mpo

unds

Fe

3(C

O)1

2 (—

201+

40)

Ru3

(CO

)12(

— 5

0/+

40)

RuF

e2(C

O)1

2 (—

30/+

40)

0s3(

CO

)12(

+ 10

0)

0s3(

CO

)12(

+ 40

)

[MnF

e2(C

O)1

2] —

(—

85/4

0)

00

Ru3

(CO

)10(

NO

)2(—

50/4

0)

Tab

le 2(

a). '3

C0

exch

ange

dat

a fo

r pol

ynuc

lear

met

al ca

rbon

yls

Che

mic

al sh

ift ö

ppm

., re

lativ

e to

TM

S A

ssig

nmen

t [r

elat

ive i

nten

sity

]

—21

2.9

—19

9.7

—20

6.4

—17

6.4

—18

2.3

[1]

—17

0.4

[1]

—22

3.1

—19

8.4

[4]

—19

6.6

[4]

—18

2.8

[2]

—25

9.8 [1

] —

214.

4 [1

0]

—22

8.8 [3

] —

183.

4[3]

—

181.

8 [3

] —

175.

5 [3

] —

190.

7

Cla

ss

Com

plet

e site

exch

ange

[HFe

3(C

O)1

] — (

— 8

0/40

)

. Tetr

anuc

lear

com

pdnd

s R

h4(C

O)1

2 (—

65°)

} axi

al-e

quat

oria

l

Mn(

CO

)4

Fe2

(CO

)8

Ru(

CO

)4

} br

idge

CO

te

rmin

al C

O

Rh—

CO

—R

h

term

inal

Rh(

CO

)3

j and

Rh(

CO

)2

} com

plet

e ex

chan

ge

axia

l-eq

uato

rial

exch

ange

rigi

d gr

oups

} ex

chan

ge w

ithou

t par

ticip

atio

n of b

ridg

e

Rh4

CO

)12 (

30°)

C

ompl

ete

exch

ange

RhC

o3(C

O)1

2 (—

85°

) —

251.

2 [1]

C

o—C

O—

Co

1 —

238.

3 [2

] R

h—C

O—

Rh

I —

201.

1 [2

] C

o(C

O)2

R

estr

icte

d ca

rbon

yl ex

chan

ge

-200

.1 [3

] C

o(C

O)3

co

rn

—19

5.5 [2

] C

o(C

O)2

J

plet

e si

te ex

chan

ge a

t hig

h te

mpe

ratu

re

—18

8.2 [1

] R

h—C

O

—18

3.1 [1

] R

h—C

O

Co4

(CO

)10[

C2(

CO

2Me)

2] (

— 10

4°)

—21

1.1

[2]

Co-

CO

-Co

—19

1.3 [4

] C

o(C

O)2

}

Res

tric

ted

brid

ging

carb

onyl

exch

ange

—

198.

2 [4

] C

o(C

O)2

Hex

anuc

lear

com

poun

ds

Os6

(CO

)18

(— 50

°)

—17

0.5

1 O

s'(C

O)

1 L

ocal

ized

site

exch

ange

at t

hree

met

ai ce

ntre

s —

176.

7 [2

] 1

176.

8 [1

] O

s"(C

O)3

(I

II a

nd I

II)

with

out c

arbo

nyl e

xcha

nge

betw

een

—17

8.6

[2] +

[1]

Os"

(CO

)3

met

al ce

ntre

s —

178

.6}

—18

2.6 [

2]

[Rh6

(CO

)151

] (—

69°)

—

245.

3

} tr

iple

bri

dge

Rig

id

—23

9.2

—23

2.9

—18

3.3

term

inal

Tab

le 2

(b).

Poly

nucl

ear c

arbo

nyls

'3C

0 ex

chan

ge

r tTl C

ompl

ete

exch

ange

R

estr

icte

d exc

haug

e ç,

Fe3

(CO

)12

Ru3

(CO

)1

I

RuF

e2(C

O)1

2 St

ereo

chem

ical

D

iffe

rent

met

al en

viro

nmen

ts

0s3(

CO

)12—

axia

l—eq

uito

rial

[H

Fe3(

CO

)1 ]

brid

ge—

term

inal

p

Hig

h co

ordi

natio

n No.

D

iffe

rent

met

als

Dif

fere

nt o

xid.

sta

tes

0s6(

CO

)18

[Mn(

CO

)4Fe

2(C

O)8

] -

Ru3

(CO

)10(

NO

)2

[Rh6

(CO

)151

] — [R

hCo3

(CO

)12]

C

o4C

O)1

0(ac

etyl

ene)

0s

6(C

O)1

8


f3-diketonate complexes16. As these possibly operate by a dipolar mechanism,the intensity of the peaks may vary depending upon the structure of themolecule. In metal systems, this possibility may be further accentuated bythe presence of nuclei which have large quadrupole moments such as cobaltand manganese.

Using Fourier transform techniques it is possible to obtain the spectra ofmany of the molecules using the l3 present in natural abundance. Incontrast to the behaviour of protonic systems this normally leads to a rela-tively simple spectrum as 13C—13C coupling is eliminated. it is convenient torelate such spectra to those of enriched 13C samples when the appropriatecoupling may be observed and utilized for assignment purposes. With theplatinum and rhodium compounds there is the further advantage that asboth these metals contain atoms with nuclear spins of -, significant metal—carbon coupling may be anticipated. As discussed below, this may be usedas a useful probe in elucidating the mechanism of fluxionality in these sys-tems. Table 2'contains data on the temperature variation of the spectra of anumber of polynuclear carbonyls and substituted derivatives. It is convenientto discuss these data in terms of the molecularity of the metal in the carbonylspecies.

(I) Tetranuclear carbonylsThe fluxional nature of metal carbonyl derivatives may be readily illus-

trated from '3C n.m.r. studies On Rh4(CO)12. The structure of the carbonylin the solid is given in Figure 2, and implies four different environments for

00 C 0

0C /\ c0\Rh

0Rh Co

Figure 2

51

J. LEWIS AND B. F. 0. JOHNSON

the carbonyl group. As stated above, the rhodium nucleus is a spin -systemin 100 per cent abundance, and so coupling between the rhodium and thecarbon of the carbonyl groups may be expected. The 13n.m.r. spectra of apartially enriched sample ('s 15 per cent), between 0° and 60° C, shows aquintet structure consistent with the scrambling of the carbonyl groupsbetween the four rhodium centres and coupling to all four rhodium nuclei20.A study of the low temperature '3C n.m.r. spectra (Figure 3) exhibited, below—65°C, four resonances of equal intensity which is compatible with the solidstate structure in Figure 21 8 In particular, the bridging carbonyl resonancewas identified as this occurred as a triplet, coupled to two rhodium atoms.Using 13 enriched samples, it was possible to identify the chemical shiftsof the three remaining carbonyl environments utilizing '3C—'3C couplingdata. The spectra were recorded at temperature intervals up to the coalescencetemperature at —5 ± 5°. The collapse of the slow exchange spectrum wassubstantially uniform, indicating that there was no site selectivity involvedin the scrambling process and may be rationalized in terms of uniform bondbreaking of the bridging carbonyls to yield a symmetrical intermediate ofTd symmetry.

Intensity (3)

Figure 3

Extension of this work to the related mixed carbonyl19 RhCo3(CO)12, thepreparation of which was recently reported by Chini and co-workers21,illustrates an afternative mechanism involving a concerted bridge breakingand making'process. The low temperature 13C n.m.r. spectra of the mixedcarbonyl [Figure 4] is completely consistent with the structure proposed byChini et a!.21. The presence of the rhodium atom in the bridging carbonylplane was established by the existence of two sets of carbonyl absorptionsin the bridging region, one being split by the rhodium nuclear spin. Theinequivalence of the two terminal carbonyls on the rhodium was also con-firmed and their identity established from the rhodium coupling. On raising

52

a

o?o\ /CJ-c\I/

Rh

75%3CO, Rh4(CO)12D2CCI2; O.OhM Cr (acac)3- 500 Coalescence

temp. -5°C

TMS

Coupling J: Rh- C :35 Hz

(3) (3) + (3)

(c+d)


a

8

+ 300

!.10°

ppm.

-30°c

I!C o

Rh CoAoB 2Rh99\

(I)

Rh

NRh COC Co A

Figure 4

53

1b'

a

-60°c

b

1(1).

b

11(2)

(2) (3)1(2) d+d' (2)I iii

- 85°c

-250 -190


the temperature, the resonance attributed to the bridging carbonyls and theterminal carbonyls of the cobalt coalesce, whilst the inequivalence of thetwo terminal carbonyls associated with the rhodium disappears but preservestheir unique bonding character to the rhodium as implied by the maintenanceof th Rh—1 3C coupling. At higher temperatures this inequivalence isremoved and a single signal, due to exchange of the carbonyl groups betweenall available sites, is observed.

These spectral data may be interpreted in terms of a concerted bridgemaking and breaking mechanism which allows the equilibration of the bridg-ing carbonyl groups and the terminal cobalt—carbonyl groups. A sym-metrical intermediate of the form considered for the Rh4(CO)12 cannotapply as this would lead also to a scrambling of the terminal carbonylsbonded to the rhodium, which only occurs in the high temperature region.This emphasizes that carbonyl site exchange is not necessarily a one-stageprocess and may depend upon the variation and disposition of the metalwithin the carbonyl cluster.

A related exchange process has been suggested'7 for the acetylene cobaltcarbonyl cluster Co4(CO), 0(RCCR), in which the 'butterfly' bondingarrangement is suggested22 for the acetylene fragments to the metal cluster(Figure 5). The low temperature spectrum of this compound shows essentiallytwo types of cobalt carbonyl environment consistent with the structure anda bridging carbonyl absorption. The inequivalence of the carbonyl groupson a given cobalt atom is also revealed in the spectra. The temperature varia-tion of the spectra has been taken to indicate bridging carbonyl exchangewith only one of the two cobalt—carbonyl atoms (C0A) and a stereospecific

FigureS

54


exchange of the bridging carbonyl with only one (COw) of the two carbonylsites of the exchanging cobalt system. This corresponds to a making of onecarbonyl bridge along a cobalt—cobalt bond (CoA__C0l3) whilst the relatedbridging carbonyl group is breaking. Thus, in the complex, the inequivalenceof the two cobalt situations is recognized in the mçchanistic process and thestereochemical inequivalence of the carbonyl groups on a given metal centreis maintained during the terminal-bridge-exchange process.

(II) Trinuclear carbonylsFor the trinuclear carbonyls M3(CO)12(M = Fe, Ru, Os), the mixed car-

bonyl, RuFe2(CO)12, and the carbonyl-nitrosyl Ru3(CO)9(NO)2, n.m.r.data are reported in Table 2. A single resonance is observed down to —40°C

Co

Fe3(CO)12 M3(CO)12(MRuorOS)

(at which temperature solubility problems arise) for the iron and rutheniumdodecacarbonyls, indicating rapid bridge—terminal exchange within the ironcomplex and rapid equatorial—axial exchange for the terminal carbonylgroups of both the iron and ruthenium complexes (Figure 6, left and right).For the osmium derivative, separation into two broad singlets is observedat low temperatures corresponding to axial and equatorial environments ofthe carbonyl groups (Figure 6). The higher energy of activation for thisexchange process in the case of osmium is consistent with the trends normallyexperienced within a transition metal triad. The behaviour of the Ru3(CO)10-(NO)2 contrasts with that observed for the parent carbonyl and exhibits adifferent exchange process to that observed in the tetranuclear Rh4 species.The '3C n.m.r. spectra show virtually no temperature variation between — 50°and + 40° and exhibit three absorption bands in the terminal carbonylregion, with an intensity ratio of 2:2:1. The structure of the compound isgiven in Figure 7, from which four groups of carbonyl absorptions wouldhave been anticipated; two groups of carbonyls associated with the tworuthenium atoms involved in the nitrosyl bridge and two environments, theaxial and equatorial distributions, for the unique ruthenium carbonylgroupings23. The data thus indicate a rapid exchange between the axial andequatorial sites of the unique ruthenium grouping whilst the two sets ofcarbonyls related to the other two ruthenium atoms remain stereochemically

55

C0

0

Figure 6


rigid. It would appear that in general if there is a significant difference instereochemistry or formal oxidation state of metals within a metal cluster,the exchange between such metal centres is the highest energy process. Thismay reflect on the nature of the bridging carbonyl groups, which are normallyinvoked as an exchange pathway between carbonyl groups on different

metal centres, and which may be very asymmetric when bridging betweendisparate metal atoms. In this respect, it is of interest to note that, in contrastto the fluxional behaviour of the bridge-terminal groups in the Fe3(CO)12system, for the related anion [Fe3(CO)1 1H], in which one of the bridgingcarbonyls is replaced by a hydrido-bridge24, two signals are observed, onein the bridging and one in the terminal carbonyl region, implying terminalexchange without exchange of the bridge carbonyl group. This indicates abridging intermediate in which carbonyl bridge formation occurs across theedge of the metal triangle that does not involve the hydride bridge and astereochemical rigidity to both the hydride and carbonyl group of the bridgingsystem.

(III) Hexanuclear carbonylsA study of the hexanuclear carbonyl, 0s6(CO)18, provides a further example

of the lack of exchange of carbonyls between stereochemically differentmetal centres, whilst exchange around a given metal centre is facile. The

56

Figure 7


structure of the carbonyl (Figure 1) indicates three types of osmium metalenvironment and two classes of carbonyl grouping on each metal centre7. The13 spectrum of an enriched 13C sample was obtained at —50°C and wasconsistent with the structure obtained for the solid. Three sets of signals,corresponding to the three metal environments were obtained and each wasseparated into a pair of signals, in the ratio of 2:1, corresponding to thevariation in the local carbonyl environment. On allowing the temperatureto rise, each set of peaks on a given metal centre coalesced to a singlet atthree different temperatures, indicating a different energy of activation for thestereochemical equilibration process of the carbonyl groups at the threeosmium sites. Up to the highest temperatures studied ('.' 100°.) no exchangebetween the metal centres was indicated.

The results obtained for the above compounds reflect on the dynamicnature of the carbonyl groups within these compounds at normal tempera-tures. However, chemically rigid systems have been observed in polynuclearaggregates; thus Chini and co-workers recently reported25 the spectrum ofthe anion [Rh6(CO)1 5]2 -, in which two sets of rhodium octahedra arecombined via carbonyl bridging groups. The detailed spectrum could beassigned in terms of a rigid structure, equivalent to that obtained for thestructure in the solid state. Variation of the temperature up to 25° showed noindication of carbonyl site exchange even on the same metal centre. Wehave observed80 a similar phenomenon for the anion [Rh6(CO)151] -, whichappears to be stereochemically rigid up to the maximum temperature studiedof 30°. In these systems, with three-centre carbonyl groups, it is importantthat there are three equivalent, three-centre groups associated within themetal framework to allow exchange to occur via an intermediate in whichthere is no electronic disparity between metal centres, the general criterionfor exchange within metal cluster systems being considered to proceed bymechanisms that do not violate the electronic requirements of the metal atoms.This may be formulated79 in what may be termed the 'Cotton principle' viz.'Carbonyl scrambling processes (i.e. those in which CO groups pass from onemetal atom to another including bridge—terminal exchange) occur only bypathways in which the electron sufficiency and electron neutrality of allmetal atoms is continuously maintained'.

For the rhodium anions, in addition to number and types of three-centredcarbonyl groups, the effective coordination number of the metal atoms withinthe cluster unit is high. The formation of a terminal to bridging carbonylgroup occurs with an increase in the coordination number of metal atomswithin the cluster unit and would therefore be less probable with increasinginitial coordination number of the metal centres involved.

(II) VARIATION IN REACTIVITY OF SOME TRINUCLEARCLUSTERS TO INORGANIC DONORS

The trinuclear dodecacarbonyls of iron, ruthenium and osmium undergoa variety of substitution and addition reactions producing compounds inwhich the trinuclear cluster is maintained. The stability of the clusterunit increases with increasing atomic weight of the metal. This may be associ-ated with the enhanced kinetic stability normally found with compounds

57


of the third row elements and/or the greater strength of the metal—metalbonds with increasing atomic weight of the metal. The determination ofmetal—metal bond energies in metal clusters has been a point of interestfor some time, and a variety of data, such as bond lengths, force constants orfragmentation patterns in mass spectroscopy, have been employed to examinetheir variation with position of the metal in the transition series. Table 3contains the most recent data for the bond enthalpies of metal—metal

Table 3. Bond enthalpy in metal carbonyls (values in kcal mol 1)

Cr Mn Fe Co NiMetal 16 19.2 22 —Carbonyl 25.7 23.7 28.1 32.5 35.1Methyl — 27.9 — — —

MetalMo—

Tc—

Ru28

Rh26.8

Carbonyl 36.3 — 41.2 39.4

MetalW—

Re30.5

Os31.1

Ir31

Carbonyl 42.6 44.8 45.8 45.3Methyl — 53.2 — —

bonds in metal carbonyl cluster systems, with the related values of thebond enthalpies of the metal—carbon bond for the terminal carbonylgroup26. It is significant to note that the increase in stability of metal—metal bonds as implied by the previous considerations, on descending atriad, is paralleled by an increase in the stability of the metal—carbon bonds.This must complicate any discussion of the stability of metal cluster unitssolely in terms of the enhancement in the stability of the metal—metal bondson descending the triad. Nevertheless, there is a marked contrast between thebehaviour of the dodecacarbonyl of iron, which generally reacts with alarge variety of ligands to yield mononuclear complexes, and triosmiumdodecarbonyl which yields mainly trinuclear complexes.

For the trinuclear dodecacarbonyls, it has been suggested that the lowestempty molecular orbital is antibonding with respect to the metal—metalbonding framework, whilst the highest filled molecular orbital is weaklyantibonding, although the absolute symmetry of the orbitals concerned is indebate" 12 Thus the addition of electrons to the metal cluster will lead to adecrease in the metal—metal character of the unit, whilst removal of electronsshould lead to an enhancement in the metal—metal bonding within thecluster. The general utility of cluster units as potential electron 'sinks' isone of the more rapidly developing aspects of polynuclear chemistry. Thegeneral chemical reactivity of the clusters is conveniently summarized inTable 4 and illustrates a range of reactions in which two metal centres aredirectly involved in bonding to the donor atoms. In the substitution reactions,two carbonyl groups, donating four electrons to the cluster, are replaced bya series of two, four and six electron donor systems. As noted above, it issignificant that iron complexes are obtained only for substitution reactions

58

M3(

CO

)1O

XY

(a

) X

= Y

= C

l, M

= O

s32

(b)X

= Y

= O

R =

SR

. R

= al

kyl a

ryl

M =

Os27

, R

u33

(c) X

= Y

= N

O, M

= Ru

36

M3(

CO

)12

+xY

I

Tab

le 4

.

-2C

0

Oxi

dativ

e add

ition

reac

tion

0s3(

CO

)12X

Y

(a)X

=Y

=C

l,Br,

127

(b)X

= C

l, B

r: Y

= P

h3A

u28

X =

Cl;Y

= S

nC13

29

Subs

titut

ion r

eact

ions

4 el

ectr

on do

nor

2 el

ectr

on d

onor

H20

s3(C

O)1

027'

(a

) M3(

CO

)12.

.L

L =

pho

spbi

ne, a

rsin

e, st

ibin

e, n

= 1

,2, 3

M

= Fe

30,

Ru3

1, O

s32

(b) M

3(C

O)1

0YX

X

= ll

;Y =

Cl,

Br,

SR

. OR

, M =

Ru3

3, O

s27'

32'3

3 X

= H

, Y =

CN

Me2

, M =

Fe34

(c

) M

3(C

O)1

0 N

SiM

e3, M

= F

e76,

Ru7

7

6 el

ectr

on d

onor

0 tn 0 z Cl) 0 r1 0 0


in which carbonyl groups are replaced with a similar class of ligands suchas phosphines, and for one class of bridged species (vide infra), whilst a rangeof derivatives are readily obtained with osmium and ruthenium. In manyinstances, however, in contrast to the behaviour of the osmium system, thetrinuclear ruthenium complexes appear as a minor component of the productdistribution with a considerable preponderance of binuclear compounds.

The oxidation addition reactions, with halogens, lead to a linear poly-nuclear system for osmium, 0s3(CO)12X2, which on heating loses carbonmonoxide to yield the triangular cluster unit 0s3(CO)10X2. The four electrondonor system of the two carbonyl groups is replaced by the six electron donorgrouping of the two halogen bridging atoms; as discussed above, this leadsto an overall reduction in the bond order within the metal framework, asthe electrons are added to a metal—metal antibonding orbital: converselythe replacement of the two carbonyl groups by two hydrogen atoms isequivalent to removal of electrons from the cluster and should lead to anincrease in the bond order within the framework. As these substitutionreactions occur with the formation of bridging species between two of thethree metal atoms of the trinuclear series, it is not surprising that this varia-tion in the metal bond character occurs in a localized manner. Figure 8

(cp)4 (Cp) (cp)4

(Cso8AOs (63A---OS0) (CFigure 8

summarizes the bond length data obtained for a series of these bridgedderivatives12. The overall metal—metal bonding varies from that of theparent carbonyl value37 of 2.85 A and is consistent with the formulation ofthe metal—metal bond orders as zero, single and double for the alkoxy,thiol and hydrido bridge systems respectively.

Of particular significance, in any consideration of these cluster units withorganic systems, is the extreme facility with which hydrogen—metal bondsare formed from hydrogen-containing compounds. Kaesz was able toestablish that these carbonyls even reacted with hydrogen gas to producehydrido carbonyls in excellent yield3 . For the ruthenium dodecacarbonylthe main product was the tetranuclear carbonyl H4Ru4(CO)12. With theosmium compound the initial product was the trinuclear adduct H20s3-(CO)10 which yielded the tetranuclear compound under more forcing con-ditions. As will be discussed below, the ready formation of metal—hydrogenlinks dominates the organometallic chemistry of these derivatives and im-plies a high thermodynamic stability for metal—hydrogen bonds in this areaof the periodic table.

The facile removal of hydrogen by these species also occurs in reactionswith water38. Thus Ru3(CO)12 reacts with water in a sealed tube at 135°C to

60


give good yields of the polynuclear hydrido-carbonyl, H4Ru4(CO)12. Therelated reaction with osmium is complicated by the formation of higherpolynuclear carbonyls, reaction in a sealed tube at 230°C yielding a variety ofproducts of varying molecular complexity. Thus the derivatives H(OH)0s3-(CO)10,H20s4(CO)13 and H40s4(CO)12 presumably arise from the reactionwith 0s3(CO)12, whilst the other hydrido species H20s5(CO)16, H20s5-(CO)1 5,H20s6(CO)18 and H20s7(CO)19C arise from the intermediacy ofthe higher polynuclear carbonyls. The presence of small traces of water inreacting systems, particularly if carried out at high temperatures, may there-fore produce intermediates of this nature and, as the hydrido-carbonylsgenerally are more reactive than the osmium dodecacarbonyl, this maylead to complications. It is, perhaps, pertinent to note that the above poly-nuclear carbonyl hydrides illustrate once more the facility of these morecomplex metal clusters to vary their electronic structure. The ligand electroncontribution in the two hydrides, H20s5(CO)16 and H20s5(CO)15, differsby two electrons; a comparable arrangement occurs for H20s6(CO)18 andfor the hydride H40s6(CO)16, which is obtained by reaction of 0s6(CO)18with hydrogen gas39.

NCo

RU\

/Ru

Ru

Figure 9

61


Another feature of these polynuclear aggregates is the ready formation ofcarbido-derivatives on controlled thermolysis of the carbonyls or the hydrido-carbonyls, often in the presence of an organic solvent. The thermal decom-position of Ru3(CO)12, in the higher boiling alkanes or as a solid, yields4°the carbido-carbonyl cluster Ru6C(CO)17; when the reaction is carried out inbenzene or related solvents significant yields of the arene-substituted deriva-tives (arene)Ru6C(CO)14 occurs in which three of the terminal carbonylgroups are replaced by the arene group (Figure 9)4041• The carbido clustersM5(CO)1 5C (M = Os, Ru) have also been prepared42 by alternative reac-tions and are similar to the iron derivative Fe5(CO)15C obtained initiallyfrom the reaction of iron carbonyls with acetylene derivatives43. The struc-ture of the iron adduct (Figure 10) contrasts the environment of the carbidogroup, which is in a facial position, with that of the hexanuclear rutheniumadduct when it is centred in the metal framework. The general chemistry ofthese carbido-derivatives is essentially unexplored; their ready formation,however, may suggest their potential intermediacy in any high temperaturereaction process involving these carbonyls.

In contrast to the trinuclear species discussed above in which interactionwith only two of the three metal centres was observed, the formation of com-pounds in which all three metal centres bond to the donor system may bereadily achieved. As may be anticipated the substituted metal carbonyls are

62

Figure 10


readily protonated in acid media44; with the ethylthiol derivative the pro-tonated species provide a route to the preparation45 of the sulphur-bridgedspecies H2Ru3(CO)9S, in which the sulphur bridges to all three metal centres(Figure 11)H Ru3(CO)10SEt -+ [H2Ru3(CO)10SEt]

+

[H3Ru3(CO)9S]-- [HRu3(CO)S]

RUI111'II1IRURu-Ru= 2.85

2.873

Figure11

As may be expected from the facility with which hydrogen abstraction occurswith these metals, this compound and the related osmium compound may beprepared more directly from the reaction of the trinuclear carbonyls withhydrogen sulphide46. These derivatives and the related selenium and telluriumcompounds47 are formally related to the paramagnetic cluster Co3(CO)9Sand the diamagnetic compound FeCo2(CO)9S prepared by Dahi and co-workers48; in these compounds the sulphur atom is considered to donatefour electrons to the bonding group framework, whilst bonding to the threemetal centres. An alternative bonding pattern for sulphur has been determinedin the compound [Fe3(CO)9S2] which was obtained from the reaction ofiron dodecacarbonyl with some sulphur donors49. The two sulphur atomsare considered to cap the two faces of the triangular grouping50. With a fourelectron donation from each sulphur, the dodecacarbonyl cluster unit hasacquired two further electrons to the bonding scheme, and the structure hasone edge of the metal triangle longer (3.37 A)and does not involve a metal—metal bond. In contrast to this, the electronically related cobalt clusterCo3(h5-C5H5)3S2 has been shown to have bicapped-suiphur bonding withessentially a D3h geometry for the Co3S2 fragment, and equal metal—metalbonds in a metal triangular unit51.

(III) THE STRUCTURE AND REACTIVITY OF SIMPLEUNSATURATED ORGANIC GROUPS WITH POLYNUCLEAR

CARBONYLS

(a) Olefins(i) Mono-olefins

The behaviour of simple olefins parallels the bonding patterns and clusterreactivity exemplified by inorganic donor systems. Thus, the reaction oftn-iron dodecacarbonyl with olefins leads to rupture of the metal clusteryielding mononuclear species in which the olefin donates two electrons to

63


the metal orbitals. In contrast, both ruthenium and osmium dodecacarbonylsreact with olefins to yield a variety of trinuclear products. With osmium, thereactions give high yields of complexes of the type (olefin)0s3(CO)9, inwhich the olefm behaves electronically in a manner equivalent to H2S withan overall donation of six electrons to the cluster unit. This involves theformation of two metal—hydrogen linkages, two carbon—metal bonds andthe donation of two electrons from the carbon—carbon double bond—abonding arrangement reminiscent of that suggested for olefm interactionwith a metal surface (vide supra). Two classes of isomers have been obtained52involving bonding of two metals to one carbon centre [Figure 12(a)] 1:1bonding, or metal bonding to alternative carbons of the olefin bond [Figure12(b)], 1:2 bonding. The x-ray structure of the ethylene—osmium adduct hasestablished a 1:1 bonding pattern for this compound53' whilst the struc-ture of the cyclo-octene-ruthenium compound involves the 1:2 structure52(Figures 13 and 14).

Ru

C(2)

/N32.29 C(1)/2/h

1 /' 292I

Os (3) 2.051 12 89

Figure13

The ruthenium olefins of the 1:2 class (cyclo-octene), were formed ingood yields from the parent dodecacarbonyl52, but the ethylene adductwas obtained55 from reaction with the hydride H4Ru4(CO)12. The majorproduct56 in this reaction was the ethylidyne compound, H3Ru3(CO)9.C.CH3. The structure of this compound is given in Figure and it is struc-

64

'H

(a)

H

Figure 12

(b)


Me

C

RU\JH ,A/ H Ru ARu -- H

Figure 15

turally equivalent to the trinuclear cobalt complexes [Co3(CO)9C.CX3], thechemistry of which has been extensively studied58. The ruthenium compoundmay also be obtained from the ethylene adduct by reaction with hydrogen56,and this is the method employed54'59 for the production of the relatedosmium compound [H30s3(CO)9C.CH3]. In the olefm and ethylidenestructures, all of the three metals are directly bonded to the organic group,but with the ethylene and cyclo-octene derivatives there is an asymmetrical

PAC—44-1-D

65

Figure 14


bonding pattern. The structures of the olefin derivatives are considered toinvolve protonation of two of the edges of the metal triangles, correspondingto the larger intermetallic distances within the triangle. This would implyan asymmetric environment for both the metal—hydrogens and the hydrogensbonding to the carbon atoms. This difference has been detected in the n.m.r.spectra for both the ruthenium and osmium derivatives of ethylene54'55 andcyclo-octene for the metal—proton environment52, but the carbon—hydrogenasymmetry has only been detected with the ethylene adducts. This is, in part,associated with the much greater complexity of the signals in the n.m.r.spectra of the cyclo-octene derivative and, in part, with the greater distanceof the centre from the metal framework.

Table 5. Coalescence temperatures for complexes (olefin)M3(CO)9

Olefin MetalMetal—hydrogen Carbon—hydrogen

T°C AG(kcal mol1) T°C iG(kcal mo11)

Cyclo-octene52 Ru —9 11.8 — — (a)Cyclo-octene52 Os 97 16.9 — — (a)Ethylene 1: Ru >100 >17.0 —4 13.4 (a)Ethylene i:i Os >115 >17.0 72 20.3(b)Ethylene 1: Ru —5 12.0 —20 12.1 (b)

(a) d8-toluenc. (b) CDCI3.

These olefm complexes provide an interesting example of fluxional be-haviour and Table 5 summarizes the data. The energy of coalescence of thetwo hydride signals associated with the metal—hydrogen bonding occur atmuch higher values than the energy of coalescence of the carbon protons,implying a different process for the equilibration of the two sites. Thelower values for the energy of activation for ruthenium compounds are con-sistent with that discussed in other systems. In contrast to the 1:1 ethylene—bonded system, the coalescence temperature of the metal—hydrogen signalsand the energy of activation of the 1:2 species is much lower. By varying thepressure of ethylene in the reaction with the hydride, H4Ru4CO)12, (>20atoms), it has been possible55 to isolate the related ethylene complex inwhich the bonding of the olefin is of a similar type to that of cyclo-octene,namely, 1:2. The energy of exchange of the two classes of protons in thiscomplex is the same, within experimental error and all these data imply adifferent mechanism for the exchange processes between the 1:1 and 1:2complexes.

For the ethylene bonding in a 1:1 fashion a number of mechanisms forthe exchange processes have been postulated. Basically, they involve eithera migration of the hydrogen atoms around the metal triangle via a bridge-terminal process or breaking of the olefm—metal bond and a subsequentrotation about the olefm bond. In the ethylene—osmium system, it has beenshown that for the complex HDOs3(CO)9(C:CH2), the C—H protonscoalescence temperature in toluene is increased by 6° without a significantrate of incorporation of deuterium into the organic group and the exchange

66


4— —

Figure 16

process is independent of concentration of the complex over the range studied.This implies that the mechanism of equilibration involves an intramolecularhydrogen migration within the metal framework6° which maintains thedifference in environment of the two metal hydrogen protons HA and HB,(Figure 16).

Further evidence in favour of this mechanism is provided by the useof the chiral centre, induced in an asymmetric olefin, on coordina-tion to the metal cluster. The two isomers of the olefin complex,H2Os3(CO)9C" :C2(CH3)[CH2CH(CH3)2], were prepared and their n.m.r.spectra recorded in deuterochloroform60. The olefm carbon(2) is a chiralcentre arising from coordination to the metal. On equilibration of the methylresonances of the group directly bonded to this carbon(2)(i.e. isomerization),the diastereotopic splitting observed for the isopropyl methyl groups remains,implying that the equilibration process does not involve the breaking of themetal—olefin bond.

For the 1:2 bonded olefin adducts the similarity in the energies of theequilibration of the two different sets of protons suggests a common mechan-ism. This may occur either by a hydride—metal shift to produce a symmetricalintermediate, by producing a terminal hydride on each of the metals -bonded to the olefm or by a rotation of the olefin around the metal cluster ashas also been suggested for the benzene adduct (vide infra).

Similar olefm derivatives have been isolated using a variety of othermono-olefins. In general, the x-olefm derivatives bond in the 1:1 manner,and in certain instances, isomerization of internal olefms occurs to yield thecz-olefm which appears to be the preferentially bonding form. Thus55 both1- and 2-cis-butene yield the same 1:1 isomer on reaction with the hydrideH4Ru4(CO)12, i.e. H2Ru3(CO)9C :C(H)C2H5.

The relationship between these ruthenium and osmium ethylidynes andthe related cobalt complexes [Co3(CO)9C.CH3] cited above, suggests aninteresting comparison between the 1:1 ethylene derivative and the 'primary'carbonium ion species [Co3(CO)9C :CHfl recently reported by Seyferth61for the related cobalt cluster. Protonation of the osmium—ethylene adduct,H20s3(CO)9CCH2, occurs in a variety of acid media to give62 the ion[H3Os3(CO)9(CCH2)]t The same cation may be prepared also byhydride ion abstraction from the ethylidyne complex [H3Os3(CO)9.C.CH3],

67


with Ph3CBFZ in liquid sulphur dioxide. The cation is easily identifiedby n.m.r. spectroscopy and is formally related to the cobalt complex reportedby Seyferth. However, below —700, two high-field signals are observed at28.36 and 30.58 t with relative intensity of 1:2. This implies an asymmetricbonding of the 'olefin' to the metal cluster and the representation of thiscompound as a metal-protonated 1:1 bonded olefm [Figure 17 (left)] ratherthan as the symmetrical carboniuin ion structure implied by Seyferth forthe related cobalt complex, Figure 17 (right).

A novel bonding pattern for benzene has also been identified in thesesystems, although it was first recognized in more complex phosphine substi-tution products (vide infra). Benzene reacts slowly with 0s3(CO)12 in asealed tube at 194° to yield54, as one of the products, H20s3(CO)9(C6H4)54.The n.m.r. spectrum indicated the bonding of the benzene as an ortho-phenylene group to two of the metals and the donation of the ic-electron tothe other metal, in a similar manner to that proposed for the phosphine-substituted adducts (vide infra). There was only one high field signal ob-served and this was interpreted as indicating a rapid exchange processbetween the two hydride metal environments consistent with the mechanismof rotation of the benzene around the metal cluster as proposed for thephosphorus-substituted species.

Benzene compounds of this type were first prepared from the controlledthermolysis of the triphenylphosphine-substituted derivatives of triosmiumduodecacarbonyl63. A complicated mixture was obtained, involving avariety of products in which not only had hydrogen abstraction occurredfrom the organic group with maintenance of the phosphorus—phenyl bondand formation of metal—carbon bonds, but also there took place the completetransfer of the 'benzene' grouping to the metal with the production ofphosphido-bridging groups, Figure 1863. The latter compounds were con-sidered to correspond to a coordinated 'benzyne' derivative, although asimplied above they may equally well be formulated as 1:2 olefm derivativessimilar to that obtained with cyclo-octene. An elegant study of the fluxionalnature of complexes of the form [0s3(C6H4)(EMe2)(CO)7] (E =As, P) hasbeen carried out by Deeming and his co-workers64. In the structure of therelated phenyl derivatives, Figure 18, the organic ring is bonded by ortho-carbons to the metal triangle and the organic ring is inclined at about 70° to

68

Figure 17


the metal ring system. Assuming a similar structure for the methyl deriva-tives, four different environments would be anticipated for the methyl andaryl hydrogens. The temperature variation of the n.m.r. spectra indicatedthat, at all temperatures studied, only two methyl signals were observed,whilst the protons on the aryl grouping showed a significant variation con-sistent with fluxional character. Two independent molecular motions wereidentified, as represented in Figure 19, corresponding to both a rotationand a 'flipping' of the organic group around the metal framework via theintermediacy of a carbonyl bridging group. The related ruthenium compoundshave been prepared65 by the thermolysis of the corresponding trinuclearruthenium carbonyl phosphine adduct. In the case of ruthenium a muchgreater preponderance of binuclear species was produced, consistent withthe highest stability of the trinuclear osmium cluster unit.

In marked contrast to the behaviour of inorganic donor systems, relativelyfew organic systems have been isolated in which bonding occurs to onlytwo of the three metal centres. One such series of compounds has recentlybeen identified from the reaction of acetylene with the dihydride, H20s3-(CO)10. A facile reaction occurs to yield the vinylidene complex H0s3-(CHCH2)(CO)10, in which a structure similar to that of the thiol-bridgedcomplex H.(SEt)0s3(CO)10 (Figure 8) is suggested66. The vinyl groupbonding by a a-bond to one metal donates an electron pair from the olefm tothe other metal atom. This complex is smoothly hydrogenated to yield the1:1 bonded ethylene complex H2Os3(CO)9(CCH2).

69

Figure 18


(Co)2 (Co)2Os Os

Me2E(/ E,Me _______ _______ Me2E0j/ C\\ //X'(CO)3O-I4Os(CO)2 (CO)2OsOS(CO)3

(ci)

(ii) Oligo-olefinsThe major studies with oligo-olefins have been restricted to ruthenium

derivatives, using either the tetranuclear hydride carbonyl, H4Ru4(CO)12 orthe ruthenium dodecacarbonyl as the starting materiaL Reactions withnon-conjugated dienes, such as 1:5 cyclo-octadiene lead52 to a variety ofproducts with H4Ru4(CO)12. Thus, a tetranuclear species involving a'butterfly' bonding pattern, similar to that obtained by reactions of acetylenicderivatives with cobalt carbonyl, is readily obtained. The structure67 of thecompound Ru4(CO)11C8H10 is shown in Figure 20. Various trinuclearderivatives are also produced in this reaction; one of the major componentsis a monohydride HRu3(CO)9C8H11, in which the organic group is bondedby a ir-allyl grouping to the metal. This complex reacts smoothly with hydro-gen to yield the cyclo-octene derivative described above, H2Ru3(CO)9-(C8H12); the cyclo-octene adduct is also produced52 by hydrogenation of thetetranuclear carbonyl cluster Ru4(CO)10C8H10.

In contrast to the behaviour with the hydrido-carbonyl H4Ru4(CO)12 1:5cyclo-octadiene reacts with Ru3(CO)12 to yield, as the main product, themononuclear compound (cyclo-octa-1,5-diene)-ruthenium tricarbonyl. Thiscomplex provides a useful starting point for the preparation68 of mono-nuclear diene systems by displacement reactions, as, in general, reactionswith organic dienes lead to the formation of trinuclear clusters. The reactionof the conjugated 1:3 cyclo-octadiene with the ruthenium dodecacarbonylyields a mixture of products which include the it-allyl derivative, HRu3(CO)9-C8H11 obtained in the 1:5 octadiene—H4Ru4(CO)12 systems and a dihydridoderivative H2Ru3(CO)9C8H10 in which one of the double bonds is coordin-ated as a 1:2 olefm and one remains uncoordinated52.

In general, the conjugated diene systems readily react with either thehydrido-carbonyl, H4Ru4(CO)12 or the parent carbonyl to yield ic-allylcomplexes. Thus butadiene yields the crotyl derivative, HRu3(CO)9C4H5, in

70

Figure 19


1.5/.

.56

Figure 20

ORU0°oc

Figure 21

71


which two metal—carbon cr-bonds are formed to the 1 and 3 carbon atomsof the it-allyl group and the it-electrons are donated to the remaining metalcentre. The x-ray structure7° of the related adduct formed from cis—trans ortrans—trans-hexa-2,4-diene is given in Figure 21. The metal hydride is con-sidered to bond to the Ru(1)—Ru(2) edge, consistent with the longer bondlength. The 13C n.m.r. spectra show that for the carbonyl groups, only oneset of resonances, associated with the ruthenium atom Ru(3), is not affectedby proton decoupling, also implying the bonding of the hydride to the Ru(1)—Ru(2) system.

The ic-allyl derivatives may be formed also by hydrogen abstraction frommono-olefins. Thus 1-butene reacts with Ru3(CO)12, to yield, amongst otherproducts, the same crotyl derivative, HRu3(CO)9C4H5, obtained55 from thereaction of butadiene with either H4Ru4(CO)12 or Ru3(CO)12. Hydrogena-tion of this compound gives the 2-butene coordinated cluster, H2Ru3(CO)9-(CH3C=CCH3)55. The latter compound cannot be prepared directly from2-butene, as this isomerizes to (presumably) the 1-butene in the presence ofthe ruthenium carbonyls, yielding a mixture of the crotyl derivatives and

the 1:1 olefm adduct, H2Ru3(CO)9(CC(Ii \55'-'2 15)

Ph 21 Ii

RuRuJ- Ru2IH"I

(b)

(a) (b)Ru1-Ru2 2.921 2.911. ARu1-Ru3 2.773 2.773Ru2-Ru3 2.779 2.776Ru1-C3 2.01. 2.08Ru2-C1 2.01. 2.10Ru3- C2 2.15 2.36

Figure 22

Studies with more complex polyene systems also involve the formation ofit-ally! derivatives. Thus, the reaction of cyclododeca-l,5,9-triene withRu3(CO)12, gives as its main product ('.' 70 per cent), the it-ally! bondedspecies whose structure has been determined71 by x-radiography, Figure 22.This structure is closely related to that reported72 for the complex obtainedby reaction of phenyl lithium with ruthenium duodecacarbonyl Figure 22(a).

72

(a)

SeLected parameters:


With cycloheptatriene, ruthenium dodecacarbonyl yields a trinuclearspecies involving two cycloheptatriene moieties, Ru3(CO)6(C7H7)(C7H9),the structure73 of which is represented diagrammatically in Figure 23. Thismolecule involves a dienyl group bonding to one ruthenium centre, with adelocalized bonding pattern between the C7H7 ring and two of the rutheniumatoms. Detailed n.m.r. studies over a temperature range have been able toestablish the fluxional nature of both ring systems. The cycloheptadienylgroup, C7H9, yields a temperature-variable spectrum, but the C7H7 ringis fluxional down to the lowest temperatures studied (' — 100°C)74.

0Ru

OC / \ CORu\

OC \/ COFigure23

It is apparent from this description that the interaction of olefm groupswith a series of metal centres provides a completely different series of bondingpatterns to that obtained at a single metal centre. The various classes ofbonding are summarized in Table 6. A study of these systems also emphasizesthe dynamic nature of the groups coordinated to a metal cluster grouping.Comparable studies on acetylene systems produced evidence for the multi-bonding character of unsaturated organic groups to polynuclear aggregatesbut, in contrast to the behaviour with olefms, the pattern for osmium andruthenium is much closer to the classical study of these systems with the irontrinuclear carbonyls, which was well documented from the work of Hübeland co-workers75.

For the ruthenium and osmium carbonyls, the stability of the trinuclearspecies is once again apparent. Reaction of methyl acetylene8' with thehydrido-osmium carbonyl, H20s3(CO),0, yields, amongst other products,approximately 40 per cent of three isomeric compounds 0s3(CO),0(C3H4)2which appear to be related to the dimeric iron complex isolated by HUbe175,and shown in Figure 24. The complexes contain the metal 'quinone' structureand are formulated on the basis of n.m.r. and infra-red spectroscopy as iso-mers of the type depicted in Figure 25.

One mechanism for the ready polymerization of acetylenes, induced byiron dodecacarbonyl, has been elegantly illustrated from the structural

73

0U

4 0

I I


'__\IIU0U

0

C

E

00z

74


R/ \/R

O CO

Figure 24

(C,O)3Os

R' R Me= R"=Me

R'"= Me

Figure25

studies of intermediates. Figure 26 shows the structures of the various com-plexes involved in the dimerization of diphenylacetylene75. An importantdifference between acetylene and olefm reactions is the ease with which twoacetylene groups are added to the trinuclear cluster: no trinuclear olefinaggregates are known, corresponding to the 'violet' isomer. This may reflectthe 'saturation' of the metal bonding system for hydrogen on interaction withone mole of an olefin. However, the close relationship between 1:2 bondedolefin complexes and acetylene adducts has been shown78 from the reactionsof ruthenium carbonyl with diphenylacetylene in alkaline aqueous methanol(Figure 27). This produces the dihydrido-carbonyl complex H2Ru3(CO)9-C2Ph2, in which the organic group is bonded as a 1:2 olefin. Reaction of thiscomplex in petroleum ether with excess of diphenylacetylene yields a com-plex, Ru3(CO)8(C2Ph2)2, which is the ruthenium analogue of the violet iron

75


(b) C6H5C2C6H5Fe3(CO)9

+ C6H5C2C6H5 - CO

C6H5

H5C\C 0C°

ocN"F

c


If, indeed, the limitation for multiple interaction with olefins is the hydro-gen acceptor power of the unit, it is of obvious interest to extend studies tohigher polynuclear systems where more than one 'triangular cluster' isavailable and more than one olefm may be accommodated. It is significantthat reactions of olefins with higher polynuclear aggregates, e.g. 0s6(CO)18,leads initially to products, presumably of a similar nature to those obtainedin the trinuclear systems, but results on more protracted interaction in theformation of polymerization products of the olefin, still bonded to the metalcarbonyl aggregates8 and involving molecular Units of C4, C6 and C8fragments.

It is of general importance to recognize that much of the chemistry inthese polynuclear aggregates differs very significantly from that of the simplemetal systems; the bonding patterns exhibited, if not providing an exactequivalence with the behaviour of organic species on metal surfaces, illustratesa new range of interactions which differs from that observed for mononuclearcomplexes. These reactions emphasize in particular the facility with whichmetal-carbon and metal—hydrogen bonds may be formed from organicmolecules, on interaction with multimetal centres.

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